Ancient Maya wetland fields revealed under tropicalforest canopy from laser scanning andmultiproxy evidenceTimothy Beacha,1, Sheryl Luzzadder-Beacha, Samantha Krausea, Tom Guderjanb, Fred Valdez Jr.c,Juan Carlos Fernandez-Diazd, Sara Eshlemana, and Colin Doylea

aDepartment of Geography and the Environment, The University of Texas at Austin, Austin, TX 78712; bDepartment of Social Sciences, University of Texas atTyler, Tyler, TX 75799; cDepartment of Anthropology, The University of Texas at Austin, Austin, TX 78712; and dNational Center for Airborne Laser Mapping,University of Houston, Houston, TX 77204

Edited by Jeremy A. Sabloff, Santa Fe Institute, Santa Fe, NM, and approved September 4, 2019 (received for review June 21, 2019)

We report on a large area of ancient Maya wetland field systemsin Belize, Central America, based on airborne lidar survey coupledwith multiple proxies and radiocarbon dates that reveal ancientfield uses and chronology. The lidar survey indicated four main areasof wetland complexes, including the Birds of Paradise wetland fieldcomplex that is five times larger than earlier remote and groundsurvey had indicated, and revealed a previously unknownwetlandfield complex that is even larger. The field systems date mainly tothe Maya Late and Terminal Classic (∼1,400–1,000 y ago), but withevidence from as early as the Late Preclassic (∼1,800 y ago) and aslate as the Early Postclassic (∼900 y ago). Previous study showedthat these were polycultural systems that grew typical ancientMaya crops including maize, arrowroot, squash, avocado, and otherfruits and harvested fauna. The wetland fields were active at a timeof population expansion, landscape alteration, and droughts andcould have been adaptations to all of these major shifts in Mayacivilization. These wetland-farming systems add to the evidence forearly and extensive human impacts on the global tropics. Broaderevidence suggests a wide distribution of wetland agroecosystemsacross the Maya Lowlands and Americas, and we hypothesize theincrease of atmospheric carbon dioxide and methane from burning,preparing, and maintaining these field systems contributed to theEarly Anthropocene.

lidar | ancient Maya | wetland agroecosystems

Precolumbian Maya civilization persisted from ∼3,000 to 1,000 or500 calibrated years before present (BP), building vast num-

bers of cities, farms, roads, and reservoirs. Over the last decade,lidar (light detection and ranging) imagery has greatly expandedour estimate of ancient infrastructure that ground survey had la-boriously identified for more than a century (1, 2). This massivebuilt environment indicates large ancient populations over a widearea with complex economies that required large-scale and diversesubsistence strategies (1, 3). Many studies since the 1970s haveused a wide array of tools to reconstruct food and farming systems,such as indigenous swidden or milpa systems and more intensiveagroecosystems on terraces and in wetlands (4). Research in the1970s (5, 6) heralded the growing evidence of ancient agricul-tural intensification, which many volumes examined in depth (7–9). Evidence has matured steadily since then for crop, soil, andwater management systems (10, 11). Wetland fields and canalsmake up one such system, and similar systems are widespreadacross indigenous Mesoamerica and the neotropics (12–19), aswell as in ancient China (20), Angkor Wat (21, 22), New Guinea(23), and in modern Africa (24). Wetland systems have been keysfor human subsistence, and their sediments and stratigraphy canprovide evidence for human responses to droughts, floods, andsea level change (21, 25–27), as well as global scale human-inducedenvironmental change that may amount to an Early Anthropocene(28, 29). Nonetheless, research on Maya wetland farming thatstarted with fervor in the 1970s (30–40) languished in controversy

and doubt after the 1990s (27, 41). This decline had multiplecauses, including uncertainty about wetland formation processes,chronology, extensiveness, and importance for Maya food pro-duction (13, 33–38). Wetlands under tropical forests are also in-herently difficult to excavate, and widespread recent plowing,draining, and deforestation are destroying many relict field systems(28). Fortunately, tropical forest cover is still extensive in someareas such as Guatemala’s Petén, where a recent Science articleestimated 67 km2 of seasonally wet, canalized fields based on lidarmapping with some ground validation beneath the forest canopy(1). These Petén field systems and canals will require further ex-tensive validation, excavation, dating, and multiple proxy evidenceto confirm their uses, chronologies, and extents over Maya history.We report here the verification of widespread ancient Maya

wetland fields in the Rio Bravo watershed of Belize based onlidar data and many excavations with multiple lines of evidencefor cultivars, formation, and chronology (Fig. 1). These ancientwetland systems occur in four main areas within the watershed.We focus on the area with the most evidence thus far, the Birdsof Paradise (BOP) fields (26, 27), to present a synthesis of forma-tion, use, and chronology based on 42 radiocarbon ages, including15 from six wetland excavations (Fig. 2 and SI Appendix, Table S1;these include all radiocarbon ages amassed for the BOP fields and

Significance

Understanding agricultural subsistence is vital for under-standing past complex societies. Lidar data are indicatingwidespread ancient Maya infrastructure. Wetland agriculturewas crucial to ancient cultures, but no previous study coupledlidar with multiproxy evidence to demonstrate the extent anduses of Maya wetland fields. We conducted a lidar surveyaround wetlands that multiple use proxies established wereancient Maya polycultural systems. Lidar indicated the Birds ofParadise (BOP) wetland field complex was five times largerthan we had previously mapped and identified an even largerwetland agroecosystem. We ground-verified the BOP fieldsthrough excavations and dating, creating a study to couplethese multiproxy data with lidar, thereby demonstratingwidespread ancient Maya wetland agroecosystems.

Author contributions: T.B. and S.L.-B. designed research; T.B., S.L.-B., S.K., T.G., F.V.,J.C.F.-D., S.E., and C.D. performed research; T.B., S.L.-B., S.K., J.C.F.-D., S.E., and C.D. ana-lyzed data; and T.B., S.L.-B., S.K., J.C.F.-D., S.E., and C.D. wrote the paper.

The authors declare no competing interest.

This article is a PNAS Direct Submission.

This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).1To whom correspondence may be addressed. Email: beacht@austin.utexas.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1910553116/-/DCSupplemental.

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canals for this project). The extensiveness of the field systems basedon lidar and the multiple lines of evidence satisfies the doubts thatled to the decline of research on Maya wetland agriculture. Weprovide clear evidence of ancient Maya construction, subsistence,and chronology along a river system connecting scores of ancientMaya centers and the Caribbean coast beyond. These newly verifiedwetland fields also add to the growing evidence for early andsometimes intensive human use of tropical forests (42), tropicalwetland extensiveness (43), and temporal correlations with vitalquestions of Maya civilization. For example, the formation and useof the wetlands coincides in time with the Maya Late Preclassic(2300–1700 BP) droughts, Late Classic population expansion(1400–1120 BP), and Terminal Classic droughts (44–47) and theTerminal Classic abandonment and major population realign-ments (∼1000–500 BP) (48). Both the Late Preclassic and TerminalClassic are periods of major population and trade-network realign-ments, and both have growing evidence for significant droughtperiods. Our findings here also begin to place Maya agricultureinto a framework of Late Holocene global change (29) based onthe expansion of wetland fields from ∼1800 to 900 BP throughburning and canalization and, thus, their greenhouse gas (CO2and CH4) emissions.Research from the 1970s through 1990s on wetland fields 40–

50 km north and closer to sea level (5 meters above sea level[masl] and below) than the current study area (∼8.1–35.3 masl)suggested chronologies from the Archaic period before ∼3000 BPto the Terminal Classic period ∼1000 BP. Conflicting resultscreated still-unresolved debates about these chronologies, naturalversus anthropogenic formation factors, and whether wetlandsystems were widespread (30–41). Research since 2001 on the RioBravo watershed has revealed different geographies, timing, anduses of ancient Maya wetland fields and canals (26–28). Theserelict field complexes cluster around ancient Maya noneliteresidential groups that occupy small rises (20–40 masl) above thewetlands. Larger Maya cities occupy the higher adjacent escarpments(∼150 masl), and many show evidence of imports from mol-lusks, turtles, and fish, which we have found in abundance inancient wetland fields and canals. Intensive excavation along

with earlier, passive remote sensing reported evidence forchronology and past uses of a small fraction of these wetlandfield systems (26–28).

Rio Bravo Wetland ComplexesThe ancient wetland farming systems occur in four main zones ofthe Rio Bravo watershed (Fig. 1). In the upper watershed (Fig.1A), investigations of the ancient Maya village of ChawakBut’o’ob suggest that intensive terrace and wetland field systemsarose and declined in the Late/Terminal Classic (1200–1050 BP)(49). Near the Rio Bravo’s mouth in groundwater fed wetlandsaround the Maya residential group of Chan Cahal (Fig. 1D),maize agriculture started in the Archaic (∼4000 BP) (26) andintensive, wetland agriculture arose as early as the Late Preclassic(∼1800 BP) although most evidence here dates from the Late/Terminal Classic (50, 51). In the lower middle Rio Bravo flood-plain, previous work indicated the BOP area (Fig. 1C) mainly alsoarose during the Late/Terminal Classic and declined by the TerminalClassic and Early Postclassic (1300–900 BP), paralleling the laterhistory of two adjacent ancient Maya centers (Fig. 1) (26, 27, 52). Wealso report a rediscovered wetland field complex (Fig. 1B). All of theexcavations discovered exclusively ancient Maya artifacts except atChan Cahal (Fig. 1D), where modern cultural items occurred in theplow zones of deforested pastures.

ResultsLidar Mapping and Areal Extent. For decades, our research grouphas used many types of passive remote sensing with little successto discern wetland fields previously discovered from small planesand from ground surveys under forests of the Rio Bravo Con-servation Management Area. Our objectives were to test thereasons that Maya wetland research had languished, includingdoubts about whether it was a widespread phenomenon. Wehypothesized that lidar could help quantify the aerial extent ofancient Maya intensive agriculture. In June 2016, we engaged theNational Center for Airborne Laser Mapping (NCALM) tocollect airborne lidar data of the wetlands and surrounding areas(Fig. 1). NCALM employed a Teledyne Optech Titan multispectral

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Fig. 1. (Upper Left Inset) Location of study area. (Left) Sentinel-2 image overlayed with lidar-derived DEM. (Right) (E) Chan Cahal North shown using the lidarintensity image, Chan Cahal West shown using the lidar bare-earth model (D), a portion of BOP (C), and the central Rio Bravo floodplain (B) using the the lidarbare-earth model. We discuss Chawak But’o’ob zone (A) in the text.

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lidar sensor (53) onboard a Piper Chieftain (PA-31-350) aircraftflying at 570 m above the ground to map an area of 274.6 km2.The sensor lasers were fired at a combined repetition rate of525 kHz with a scan angle of ±30° and a scan frequency of 25 Hz.The processed point cloud yielded 11.065 billion returns from6.516 billion laser pulses (∼1.7 returns per pulse), on average23.7 pulses/m2 and 40.1 returns/m2 with a vertical accuracy esti-mated at of 4.4 cm root mean square error (RMSE). The classi-fied point cloud was used to produce digital elevation models(DEM) as well as active multispectral images based on the returnintensity at three different laser wavelengths (1,550, 1,064, and,532 nm).Lidar technology has been a remarkable boon to Maya archaeol-

ogy (1, 2, 53–59), and to other areas of ancient civilizations intropical forests, such as Angkor Wat (60) because of lidar’s abilityto penetrate and to map the ground through gaps in the forestcanopy. In tropical forests, lidar has helped to identify massiveancient infrastructure. This present study in the Maya regioncouples the quantification of large-scale, wetland canal complexesthrough lidar with multiple lines of verification for the types andchronology of these ancient farming systems. In addition, thisstudy complements lidar’s spatial and spectral data successfullyto document these agricultural features under different typesof land covers.Because of the topographic and spectral characteristics of

these ancient agricultural features and their varied vegetationcover, we were able to use spatial (topographic) and spectral(intensity) lidar data together. In the study area, the key sourcereports tropical forest cover is ∼77%, savanna ∼6.1%, agricul-ture ∼16.3%, and wetlands ∼1.1% (61). In regions with treecover, we were able to use spatial imagery derived from DEM tomap the topographic signatures of the features. The DEM rep-resents a bare-earth model of the surface without vegetation. Inregions of savanna and agriculture, we were able to use imageryderived from the lidar multispectral intensity (LMI) at the 1,550-,1,064-, and 532-nm wavelengths to map and identify the spectralsignatures of the agricultural features in similar fashion of whathas previously been done with satellite imagery. This represents afortunate set of conditions given that all of the previous archae-

ological lidar research in the region has primarily used the spatialdata of lidar to identify anthropogenic topographic signatures,with a few attempting to use intensity as an additional feature (62,63) but without success documenting large-scale features aspresented here.The topographic and spectral lidar imagery indicated wetland

agricultural systems at all four zones where ground survey hadpreviously discovered them. DEMs, however, helped define manymore wetland complexes at two of the sites with thick forestcanopy: BOP and Chawak But’o’ob (Fig. 1 and SI Appendix, TableS2). DEMs also helped identify a zone, the largest, in the mostlyunexplored central Rio Bravo floodplain (Fig. 1B). In addition,the LMI facilitated defining more wetland complexes at ChanCahal (Fig. 1E). Here, deforestation and growth of dense tropicalpasture grasses for the last 50 y (50) limited DEM fidelity.BOP contains the largest verified area of Maya wetland fields

visible on lidar (Fig. 3 and SI Appendix, Table S2), with a ∼5 km2

area of canals and fields including the original ∼1 km2 area ofBOP complex discovered by aerial photography (Fig. 3B). Manyof the BOP canals under canopy are visible in DEM (Fig. 3A)because laser penetration mapped the surface topography ofcanals and fields well. The BOP wetland complexes visible in thetopographic imagery include ∼71 km of 3-m-wide canals and amedian field size of ∼2,790 m2 (mean of ∼7,031 m2) (Figs. 1C and3). The field and canal elevation ranges from ∼9.5 to 17 masl.Canal length may be longer and field size may be smaller thanthese lidar estimates because we counted the area between canalsas fields although there may be undetected canals in some of thelarger fields with denser vegetation (62). Ironically, the originallydiscovered wetland complexes (Fig. 3B) (26) are less visible inDEM, although these are identifiable from ground-verified Geoeyeimagery and the LMI. Here, in dense tropical marsh, the canals arevisible spectrally although topographic expression is low, ∼0.5 m.North of the Rio Bravo’s riparian forest (Fig. 3C), canals and theonly Maya mounds yet discovered within these wetland systemswere visible from pedestrian survey but not with DEM or LMI.The DEM imagery also indicated a small wetland field complex

of ∼0.3 km2 at Chawak But’o’ob (Fig. 1A) (49) and the largestcomplex of ∼7.7 km2 in the middle of the Rio Bravo valley (Fig. 1B).

Fig. 2. AMS dates chart and conceptual model of wetland formation (SI Appendix, Table S1 for AMS ages).

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At this latter field complex, ground-verified canals lie in closeproximity to Maya architecture. These field complexes range from∼33.5 masl to ∼13 masl in the upper valley. Most wetland fieldsand canals are clearly visible from both aerial photography and thelidar DEM at Chan Cahal (Fig. 1D), a perched wetland fed byperennial springs, and at Chan Cahal East in the coastal plainfloodplain (50). These wetland complexes together are ∼1 km2

and range from ∼17.6 to 22 masl and ∼8 to 12 masl, respectively.At Chan Cahal North, from ∼17.6 to 20 masl, the DEM did notreveal most of existing canals because of thick pasture grasses andlow elevation difference of ∼0.5 m between canals and fields. TheLMI again clearly shows the canals and fields, but as the differ-ence in soil moisture in the canals versus the fields reflected in thevisible and infrared signatures of the vegetation (Fig. 1E and SIAppendix, Fig. S1). Therefore, the LMI combination of intensity ofthe first returns of each lidar pulse distinguishes canals based onspectral characteristics of the vegetation in the canals rather thantopographic differences. We also note the DEM allowed the firstlocal identification of another form of intensive agriculture in ter-race groups at Blue Creek (Fig. 1D), the region’s largest intensivelystudied ancient Maya center between La Milpa and Lamanai.The collective area of wetland complexes is 14.08 km2 within

one medium-sized watershed with a dry season discharge of ∼6–7m3·S−1. All of these wetland areas are floodplains or ground-water seeps subject to aggradation (64) and, thus, a large area ofcanals may be buried and invisible to lidar.

Field Chronology and Uses. To understand how and when the an-cient Maya used these ∼14 km2 of canal and field complexesrequires evidence of timing, use, and processes of formation.Ancient wetland canal and field systems are diverse acrossMesoamerica (13) but greatly understudied, and a key distinctionis whether they are perennially or seasonally wet (34). Possiblecanals and fields in upland bajos of Guatemala and Mexico (1, 54)

lie mostly within seasonally wet-dry areas. The BOP fields,however, are perennial with the water table near the surfaceduring the wet season and within the capillary fringe in the dryseason. Previous studies found that BOP wetland complexesformed through ancient Maya canal excavation and field raisingto manage water for water quality and quantity. Deposition fromhuman-induced erosion, natural flooding, and natural gypsumprecipitation also buried the wetland systems (26, 27, 64).Evidence for the BOP Late/Terminal Classic wetland agro-

ecosystems came from excavations of more than 17 trenches from2005 to 2018 across fields and canals, which we mapped withaerial and ground survey (26, 27) after first identifying the canalsfrom a small airplane in 2004. The BOP wetland field complexpreserves its stratigraphic records because of sediment accumu-lation in the Rio Bravo floodplain that buried the wetland fieldsand canals (Fig. 2). The surrounding BOP floodplain has beenaggrading from 0.82 to 1.92 mm·yr−1 at nearby sites over the last3,484–3,718 y (Fig. 2), but, locally, fields experienced faster ratesin the Late/Terminal Classic period, accelerated by ancient Mayafield raising from canal excavation (49). Gypsum precipitation,from groundwater with high concentrations of dissolved Ca+2 andSO2-

4, added to aggradation (65).Since lidar imagery indicated five times more wetland field

area than what was previously estimated from pedestrian andaerial photo surveys, we studied areas A and C (Fig. 3) with six newtrenches and 15 additional AMS (accelerator mass spectrometry)ages (Fig. 2 and SI Appendix, Table S1). Based on these, thestratigraphy of the clay-rich floodplain soils of the wetland fieldsprovides evidence of ancient Maya field building (Fig. 4). Here, Oand A horizons lie over Cy (gypsum rich), Cg (gleyed), and Ab(buried topsoil) horizons above earlier Cg horizons. The buriedsoils occur in field zones, between the canals, and lie belowwidespread ash layers from burn events preceding field and canal

Fig. 3. BOP field areas (Left, A–C) with the Maya site of Akab Muclil and Center of Gran Cacao. Left has color enhancement for elevation, and Right is ashaded relief map of the DEM.

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building. The burn layers are largely continuous, except wherecanal digging erased them, and 1 to 3 cm thick with pale colors oftheir major minerals (CaCO3, CaSO4, and silicate clays) whereburning of the dark, organic matter occurred. AMS ages date tothe Late to Terminal Classic within three separate ash layers(1309–1083 BP), sandwiched by three Terminal to Postclassic(1170-800 BP) dates 12 to 20 cm above the ash and two Late toTerminal Classic (1300–1060 BP) dates within 15 cm below theash (SI Appendix, Table S1). One AMS age, which lies 180 cmbelow the surface and 10 cm below a burn layer, dates to the LatePreclassic, which is the earliest date for field burning (Fig. 4 andSI Appendix, Table S1). We interpret the zones 20 cm below andabove the ash layers as buried topsoils, Ab horizons, composedof the preburn paleosol, the ash layer, and sediment added abovethe ash (Figs. 2 and 4). The depths of this whole zone vary from72 to 180 cm below the surface (SI Appendix, Table S1). Thesefield zones also had ceramics, faunal remains, greater melanism,charcoal, and geochemical evidence (26). Above the Ab horizon,most AMS ages become younger in the aggrading floodplain overthe last millennium, whereas below the Ab, most radiocarbon agesbecome older.Canal sections (Figs. 2 and 4) provide evidence of their last

uses and deposition after disuse. The canal surfaces are today20 to 50 cm lower than field surfaces but were ∼100 cm lowerwhen they functioned, having filled faster after abandonmentdue to higher trap efficiency. Many canals still flow in the wetseason. The canal soils usually have O horizons or A horizons builtatop layered organics, clays, and silt or sand layers with precipitatedgypsum. Canal zone excavations often reveal layers of running andslack water deposits, lying above mélanges of the last Maya usesmixed with subsoils (Cg horizons left behind from soil profiletruncation by ancient Maya canal digging). A series of radio-carbon samples across this boundary shows Terminal to Post-classic ages through the depositional canal sediments (Figs. 2and 4). The buried fields and canal bottoms produced the onlyartifacts (ceramics and lithics); the few diagnostic ones date to

the Late or Terminal Classic. Some wood artifacts appear in thePostclassic-aged sediments above the canal bottom zone, and thenearby ancient Maya centers of Akab Muclil and Gran Cacaohave some Postclassic artifacts (52).Other lines of evidence for the uses of these field complexes

includes pollen, phytolith, charcoal, macrobotanical, and faunal.Previous investigations from Maya period regional wetlands in-dicate multiple ancient food species such as maize (Zea mays),avocado (Persea americana) and other fruits, squash (Cucurbita spp.),cassava (Manihot esculenta), and arrowroot (Maranta arundinacea) aswell as Gossypium spp (26, 27, 65–67). Multiproxy sedimentcores obtained within 1,000 m of the fields also produced Classicthrough Postclassic maize pollen (52). Faunal remains also in-dicate widespread protein harvesting. Burned animal bones andshells, especially the mollusk genera Pachychilus and Pomacea,were common in excavations. Another line of evidence for howthe Maya used these field systems are carbon isotope ratios (δ13C)from dated profiles of the soil’s humin fraction that providegeochemical evidence of vegetation because plants using the C3photosynthetic pathway dominate the area today while the mainC4 plants here are maize and weedy species associated withhuman disturbance. Synthesized findings from four canals andtwo field zones show the δ13C rising from the C3 plant signatures(∼−28‰) at the surface to as much as 64% derived from C4plants (∼−12‰) in the Late Classic sediments, which also havemaize pollen and macrobotanical evidence, before returningback to C3 signatures in older, lower sediments (26–28, 49).The watershed’s three other wetland field zones near the

nonelite centers at Chan Cahal, Akab Muclil, and Chawak But’o’obprovide nearby comparisons (Fig. 1). At Chan Cahal, near themouth of the watershed, phytoliths and macrobotanicals from aLate Classic structure (∼1200 BP) indicated squash (Cucurbitaspp.), maize cobs (Zea mays), beans (Phaseolus spp.), cacao(Theobroma cacao), and sapodilla (Manilkara zapota) fruit (50).Wetland agriculture may have started in the Late Preclassicbased on maize pollen from this time (2140–1870 BP (26, 50))

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and persisted into the Postclassic (after 1000 BP) at Chan Cahal(50) and Akab Muclil (52), but most agricultural evidence fromthe canals and fields dates to the Late and Terminal Classic (26–28, 50). At Chawak But’o’ob, in the upper end of the watershed,evidence for wetland farming in the Late and Terminal Classicincluded the surface expression and stratigraphy of canals andfields, copious charcoal, and carbon isotopic evidence of C4species. Here, the δ13C from soil humin rose through the Classicsediments by 6‰, an increase of ∼37% in C4 plants in a regiontoday virtually without C4 plants (49).Taken together across the watershed, these multiple lines of

evidence, coupled with the lidar imagery, indicate large-scale,ancient, polycultural wetland agricultural systems, especially inthe Late, Terminal, and Early Postclassic.

Conclusions and DiscussionWe confirmed our hypothesis that lidar data could identify morewetland field complexes under canopy, estimating ∼14 km2 in asmall watershed. Moreover, our original dating from before lidarholds well for the new areas that we excavated and dated basedon lidar (SI Appendix, Table S1). Most of the BOP and otherthree wetland field zones have dating consistent with the Lateand Terminal Classic with some Postclassic evidence (26–28, 50).We presented evidence of Late Preclassic burning in the BOPcomplex, which chronologically coincides with evidence at ChanCahal (50). For the Rio Bravo watershed, at least, we show thatwetland agroecosystems were extensive and persistent, whichcounters the doubts that had caused research to languish in the1990s. Our findings parallel evidence from 40 km farther north inthe Belize coastal plain that these systems lasted late in ancientMaya history (41).This large area of intensive farming has several implications.

First, the role of wetland farming in Maya subsistence requiresgreater attention, especially since other studies may show largeareas of canals and fields (1, 54). This article on northwesternBelize provides strong evidence for chronology and crops andsuggests integration with populations and trade routes, but weneed these lines of evidence from many more field complexesacross the Maya world. Second, the wetland fields expand in theLate Classic, a time of regional urban population increase (48,68, 69). Along slopes at this time, the Maya were also buildingother forms of intensification—agricultural terraces (28). Thelarge size of the BOP and surrounding fields may reflect local useor longer-range trade. The fields sprawl around two ancientMaya sites including the extensive but little-studied Gran Cacao(70) and other nearby centers (52). The BOP fields had access toboth human tumplines and canoe transport through a complexnetwork of rivers and canals. By canoe, the BOP system linked tothe Rio Hondo, Chetumal Bay, and the Caribbean, knowntrading routes of agricultural products (66). This large-scale ag-ricultural expansion may have been a response to a demand forfood production or perhaps for commodities and specialty items.The BOP wetlands produced several crops as well as mollusks,but we have yet to find cacao, a high-status crop that wouldconfer some possible elite connections to the wetland fields.Excavations at Chan Cahal, 10 km north of BOP, however, re-covered evidence of cacao and more than 200 pieces of jade,although of lesser quality than at the elite sites (66).Third, evidence suggests most field and canal construction was

coincident with two major environmental changes: water tablerise associated with sea level rise (26, 27, 64) over the MayaPreclassic and Classic periods and droughts during the Late/Terminal Classic and Early Postclassic (46) (Fig. 2). Especiallyduring the Maya Classic population expansions, water tablesinundated these field systems, to which the Maya adapted bybuilding wetland field complexes. Later, the Terminal andPostclassic were times of widespread population abandonment inthe central Maya lowlands (69, 71) but the wetland fields persist,

perhaps providing subsistence oases as uplands far above thewater table became riskier. We note that most canals start toinfill with no dredging in the Terminal Classic and Early Post-classic, but some show continued use while the Precolumbiancenters of Akab Muclil and Gran Cacao along the margins of BOPdid have Postclassic evidence (72). We also note that the largeMaya center of Lamanai, just 25 km east, had a long Postclassicoccupation (73). The wetland field environment, during regionallydry conditions, may have reattracted or maintained less intensiveland uses like hunting, fishing, gathering, and milpa farming.Lastly, the vast agricultural infrastructure of Maya civilization

that lidar indicates in this study and possibly in others (1, 2) addsto the growing evidence for earlier, more intensive, and morewide-ranging anthropogenic impacts on tropical forests (42, 74)and wetlands (20). We also hypothesize that these activitiesadded atmospheric CO2 from burning to build the wetland fieldsand CH4 from wetland creation through canal and wetland ex-pansion (29). Studies (20, 75), for example, mapped the evolu-tion of paddy rice agriculture in China, which coincides with therise of 70 ppb of CH4 over the last 5,000 y. The largest pre-modern increase of CH4 (∼60 ppb) from 2000 to 1000 BP co-incides with the rise of Maya wetland agriculture and likely amuch larger area of neotropical wetland canals and fields (24,76). The discernible area from the lidar imagery of this study is∼14 km2 in one drainage basin, and the area is probably largerbecause modern plowing, aggradation, and draining havemasked other canals. Other recent studies have identified addi-tional, possible wetland field complexes in northern Belize (77),Guatemala’s Petén (1, 63), and Mexico’s Yucatan, which may bemuch larger than those in this study (54). However, multipleproxy research will need to confirm their areal extents, quantifytheir greenhouse gases, and identify their uses over time to ex-plore their possible global roles in the Early Anthropocene.

Materials and MethodsLidar Data Collection, Processing, and Analysis. For this research, NCALMcollected Lidar data between July second and fourth of 2016 for an areacovering ∼275 km2 (Fig. 1) employing the Teledyne Optech Titan MW mul-tispectral lidar (53). The collection was performed from a height of 570 mabove the ground with each laser channel firing at 175 kHz (total 525 kHz)and the scanner oscillating at ±30° and 25 Hz. The processed point cloudyielded 11.065 billion returns from 6.516 billion laser pulses (roughly1.7 returns per pulse), on average 23.7 pulses/m2 and 40.1 returns/m2. Avertical accuracy assessment based on 2,238 checkpoints collected via kine-matic GPS along a stretch of open road within the project area yielded aRMSE of 4.4 cm. The point cloud was classified and a variety of rasters weregenerated based on the point cloud including half-meter resolution firstsurface (digital surface model [DSM]) and bare-earth (digital elevationmodel [DEM]) models as well as multispectral Lidar intensity images.

The lidar sensor, a Teledyne Optech Titan MW (S/N 14SEN/CON340),operates three lidar channels with different laser wavelengths and lookangles. Channel 2 resembles a traditional single-channel nadir-looking lidarsystem, and it is based on a 1,064-nm laser (near infrared) scanned with anoscillatingmirror on an arc of±30° from nadir. Channel 1 is based on a 1,550-nmlaser (near infrared), and it is scanned through the same mirror and scanangle of channel 2, but it is pointed 3.5° forward of nadir. Channel 3 oper-ates a 532-nm laser (green) scanned with the same mirror and angle butoriented 7° forward of nadir. Each channel can operate at pulse repetitionfrequencies (PRF) of 50–300 kHz, for a total combined (and synchronized)PRF of 150–900 kHz (53). For this project, two GPS stations were temporarilyinstalled while the aircraft was flying, one station was located in the out-skirts of Belize City (17.513388563° N, 88.224689048° W −1.419 m) and asecond one was located at the Maya Research Program camp near BlueCreek (17.888648859° N, 88.923225625° W, 157.322 m).

Lidar flight planning was based on a nominal laser shot density of 24 laserpulses per m2 with a configuration that ensures high penetration throughthe forest canopy based on experiments performed by NCALM in similarenvironments. The nominal flight parameters consist of a flying height of570 m above ground level (AGL) and a ground speed of 70 m/s and a lateralswath overlap of 50% (edge of swath overlaps the center of the adjacentswath). The lidar instrument was configured with a PRF of 175 kHz per

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channel for a total combined PRF of 525 kHz. The scanning mirror was set atfull motion of ±30° at a frequency of 25 Hz. While the instrument config-uration is usually kept constant during a survey, the flying parameters varydue to topographic relief and winds conditions, and this will cause local andglobal variations on the actual laser pulse density with respect to thenominal plan.

Once the point cloud has been cleaned and classified different kinds ofrasters are generated at 50-cm node spacing based on both the elevation andthe spectral (intensity) data of the lidar returns. The elevation rasters includethe first surface or DSMand the bare-earth or DEM according to the Americanusage of the term (the equivalent European usage is digital terrain model orDTM). The spectral or intensity rasters are derived from the first returns ofeach lidar channel (1,550, 1,064, and 532 nm) and are four different rasters,three individual gray scale images for each channel and a fourth raster wherethe individual intensity images are combined into a three-band false colorraster (red 1,550 nm, green 1,064 nm, and blue 532 nm).

The point cloud interpolation source files are gridded tile by tile usingSurfer by Golden Software, which is run in batch processing mode using ascripter. Both DEM and DSM tiles are generated by interpolating the sourcepoint clouds using the Kriging algorithm. The only parameter value differ-ence used to generate DSM and DEM is the Kriging search radius. For denseDSM source data, the search radius is set to 5 m, while for the sparse groundreturn source data used to build the DEM, the search radius is set to 20 m. Forthe production of the intensity rasters, three different sets of source fileswerecreated with three attributes: the X and Y coordinates and the return’s lidarintensity for each first return, but separated per tile and per channel. Theintensity rasters are interpolated using the inverse distance weighting algo-rithm based on a quadratic power and a search radius of 3 m. The individualsurfer grid tiles are then mosaicked into larger raster datasets using the “GridMosaic” tool in Surfer, while at the same time they are exported into theArcGIS raster float format (*.flt). Further information on NCALM lidar dataprocessing workflows and considerations can be found in Fernandez-Diazet al. (78).

Standard shaded relief images (azimuth 315°, elevation 45°, Z factor 1)were generated for the DSM and DEM in ArcGIS using the Hillshade tool inthe Spatial Analyst Toolbox. Many of the wetland agricultural features weredirectly visible in the shaded relief images; in some other areas they werenot as easily identifiable. Advanced lidar data visualization techniqueswere tried employing the Relief Visualization Toolbox (RVT) developed bythe Research Centre of the Slovenian Academy of Sciences and Arts. Wealso used Simple Local Relief Models (SLRM) to enhance visualization ofthe agricultural features.

The geodetic elevations for the lidar products are referred to the WGS84ellipsoid. Because a few geoid models exist for the study area, to convert therelative elevations of the produced DEM to elevation above mean sea level,we selected the Earth Gravitational Model 2008–WGS84 version (EGM2008)(79). The EGM2008 provides geoid undulation values (N) relative to theWGS84 ellipsoid at 2.5-min resolution for the globe. These values can beused to convert the ellipsoidal height (h) to orthometric height (H) in metersabove mean sea level with the simple formula H = h − N. First, theEGM2008 was extracted for just the area covering the lidar swath andresampled to 0.5-m resolution to match the DEM. With both datasets in GCSWGS84, we applied the formula to produce a DEM in meters above meansea level. For estimating the elevation ranges of the fields, we also removedextreme values using the “Fill” tool in ArcMap 12.2 to fill the sinks inthe data.

Field and Canal Metrics. We delineated wetland canals and field extents on ageographic information system using a combination of visualization tech-niques of the lidar data (SI Appendix, Tables S2 and S3). First, we overlaid ahillshade model at 80% transparency on top of the DEM with a color pal-ette, which provides elevation data, but with the realistic topographic tex-ture from the hillshade. Although this visualization is very useful, we knewfrom ground surveys that not all of the canals were visible with this tech-nique. Next, we derived a SLRM using the RVT (80). The SLRM accentuatesconvex versus concave features in the landscape, more clearly revealingmany of the canals that were not clear in the hillshade and DEM combina-tion, such as at Chawak But’o’ob. We tested other products from the RVTcommonly used in archaeological identification such as sky-view factor andPCA (62, 63), but these proved less useful than SLRM for identifying canals. Inareas with tall (2 +m), dense grasses such as some of the savanna, the DEM isless well-defined and the surface appears rough because of the difficulty ofseparating vegetation from ground returns. Thus, canals under this type ofvegetation are not visible in the DEM-derived visualizations. Lastly, the in-tensity data from the different wavelength lasers were viewed individually

as grayscale images, and together as an RGB composite. The red and nearinfrared wavelengths most clearly defined ancient Maya canals that werenot apparent in any of the other visualization methods, such as at ChanCahal North and Chan Cahal East. Although the intensity images clearlyshow these canals that are not visible in the DEM, they only reveal those thatare not under dense canopy but in the open modern agricultural fields andsavanna. The different data products (topography and spectral intensity)are, therefore, complementary under different environmental conditions.The multiple visualization methods allowed for a comprehensive inventoryof canal systems that multiple analysts were able to digitize. The linearvectors were buffered to 3 m wide to reflect average canal width based onfield measurements and the lidar-derived DEM. These vectors provide thetotal canal length for each of the wetland field complexes. The vectorproduct was then converted to a raster in order to classify distinct fieldsand canals. Lastly, the raster was converted back to a vector in orderto calculate individual field dimensions and summary statistics for thesystems.

Dating.We report all 42 radiocarbon ages, including 15 new and 27 previouslyreported AMS radiocarbon analyses from samples of burned and not burnedorganic material in discrete strata collected from our field and canal exca-vation units in the BOP complex (Fig. 1 and SI Appendix, Table S1). BETAAnalytics provided ages for 18 of these samples, International ChemicalAnalysis for 13 samples, and The University of Arizona’s AMS Lab providedages for 11 more samples. Conventional ages are in years BP (BP = BeforePresent, 1950 AD), and the AMS laboratories calibrated ages usingINTACAL13 and corrected all ages for fractionation. Here we report all agesusing 2-sigma calibration (95% probability).

Carbon Isotopes. Geoarchaeological studies in Central America have mostfrequently measured δ13C in the humin fraction of the soil organic matter, asit is the most stable part of the soil carbon (81, 82). Because of the stability,this fraction is the most likely to represent the vegetation of the past andhas more potential to reveal the C4 signature of ancient maize cultivation(82). The humin fraction of the soil was extracted using a process used inprevious studies of the region (81) in the Soils and Geoarchaeology Labo-ratory at the University of Texas at Austin. The process begins by weighing2 g of sample homogenized to pass through a 100-mesh sieve (149 μm) intoa 100-mL test tube and adding 1 M HCl until effervescence stops. The tubesare then placed in a 70 °C water bath during the reaction to assure calciumand magnesium carbonates are removed. Next, the samples are transferredto 50-mL Oakridge Centrifuge tubes and centrifuged at 10,000 × g for30 min and the supernatant discarded. The samples are then twice rinsedovernight with deionized water and centrifuged. We then use alkaline py-rophosphate extraction (0.1 M sodium hydroxide and 0.1 M sodium pyro-phosphate) to remove the humic and fulvic acid fractions from the samples.About 25 mL of the solution is mixed into the centrifuge tube with thesample, capped with a septum cap, and the headspace is flushed with ni-trogen gas to remove oxygen and prevent oxidation of the humic acids. Thesamples are shaken in the alkaline pyrophosphate solution on a mechanicalshaker overnight and centrifuged at 30,000 × g for 2 h. This step of addingalkaline pyrophosphate, shaking, and centrifuging is repeated 3 times to-tal, discarding the supernatant each time. We then rinse the samples withwater, with 0.05 M phosphoric acid to remove the added alkalinity, andthen one more time with water. Finally, the samples are oven dried at105 °C and the pellets are crushed with a mortar and pestle and arepacked into tin capsules. We analyzed the samples using a Leco ElementalAnalyzer and Isotope Ratio Mass Spectrometer in the Jackson School ofGeosciences at the University of Texas at Austin. The samples are com-busted in the analyzer and converted to CO2, and the various masses ofCO2 are measured to determine the ratio of 13C to 12C, as well as the totalorganic carbon. The results are reported as δ13C in ‰ as compared to thestandard PDB (Pee Dee Belemnite).

ACKNOWLEDGMENTS. We thank the Belize Institute of Archaeology, theProgramme for Belize Rio Bravo Conservation Management Area, and theBlue Creek Community, Belize; the National Center for Airborne LaserMapping; The Northwestern Belize LiDAR Consortium; N. Brokaw, M. Canuto,K. Cox, N. Dunning, T. Garrison, S. Houston, T. Inomata, J. McNeill, S. Ward, G.Wells,the anonymous reviewers, and our many collaborators in Belize. This work wassupported by National Science Foundation Grants 0924501, 0924510, 1114947,and 1550204; National Geographic Society Committee for Research andExploration Grants 7861-05 and 7506-03; The C.B. Smith Sr. Centennial Chair,College of Liberal Arts, and Center for Archaeological and Tropical Studies,University of Texas (UT) Austin; the Maya Research Program, UT Tyler; andthe Cinco Hermanos Chair, Georgetown University.

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